No. An engine's manifold pressure/vacuum indicates load. If you apply the same load, you get the same vacuum, no matter how much pressure is in front of the throttle blade. You're working the equation from the wrong angle. Manifold pressure is resultant. You cruise at say 65mph. At 65mph, your engine may make 12" of vacuum on a flat road, 20" down hill and 4" up a hill. You aren't after a vacuum number, you're after 65mph, and the throttle plate is what you change to achieve that goal. Your example assumes the throttle plate is in a fixed position. If it were, the denser air would pass the throttle plate and speed up the engine. Cruise speed would go up,as would fuel consumption.